The present invention relates to a drive coil protection apparatus and method for preventing drive coils from burning out as well as the dot matrix printer incorporating the protection circuit.
Printers are used as terminals for computer systems and for printing slips and receipts. Because of their low cost and relatively fast printing speed, dot matrix printers are used in many fields to print characters, numbers, etc., in dot matrix utilizing wires driven by coils. However, when the printing frequency is high, the temperature rises because of the heat generated from the coils. As a result, the print head often burns out. Thus, the temperature of the print head must be monitored frequently and inexpensively.
FIG. 8 shows an example of how the temperature is monitored in a print head in prior art. In this example, a temperature detector 19, such as a thermistor, is attached to a rear surface 10a of a print head 10. A warning sound is generated when it is determined from the changes in the resistance value of temperature detector 19 that a predetermined temperature has been reached, whereby the decrease in the printing frequency of the printer is urged to prevent burnout of print head 10.
Another example is disclosed in U.S. Pat. No. 5,042,375, entitled "Device for Temperature Control of a Print Head or of a Hammer Block Including an Electromagnetic Coil." The device detects the changes in temperature by measuring the increment in the resistance value of the head drive coil.
However, in the first example above, temperature detector 19, which is mounted on the outside of the print head, cannot accurately detect the internal temperature of the print head, particularly that of the coil which contributes most of the generated heat and is most susceptible to burnout, because of the thermal resistance of the print head case. Therefore, it is difficult to set a thermal threshold at which a warning sound, for example, should be generated. If the tolerances for the thermal threshold are small, warnings will be generated more than necessary, thus decreasing the operating efficiency of the printer. On the other hand, if the tolerances are too large, burnout may already have occurred before a warning sound is generated. Also, if a warning sound is generated and the operator fails to reduce the printing frequency of the printer, the print head will burn out. Furthermore, a space around the print head is needed for mounting the thermistor or other temperature detection element. However, in recent years the compactness of printers has been emphasized and it is desirable that a temperature detection means can be disposed within a compact printer.
In the second example above, the drive coils in different print heads have different tolerances in the thermal-resistance character. The tolerances depend on the physical dimensions of the drive coils and are caused by various factors in the manufacturing process. Therefore, the print heads are classified based on the thermal-resistance character of the drive coils. Alternatively, the resistance thresholds of the drive coils, at which the heat reduction sequence starts, are changed according to the thermal-resistance character of drive coils in order to protect the drive coils. However, mass-productivity is reduced by the classification of print heads or the alteration of the thresholds.
To solve the above problems, the invention provides a drive coil protection apparatus and method and a printer incorporating the apparatus that can prevent burnout of the print head without reducing the printing efficiency and the mass-productivity.